1. Field of the Disclosure
Embodiments disclosed herein relate generally to the melt processing of polymers of monovinyl aromatic compounds and rubber modified polymers of monovinyl aromatic compounds. In a more specific aspect, embodiments disclosed herein relate to the melt processing of microwave-heatable high impact polystyrene compositions.
2. Background
Polystyrene is one of the largest volume thermoplastic resins in commercial production today. Unmodified polystyrene is well suited to applications where its brittleness is acceptable. Engineering plastics have been used in applications where impact resistance is required, but such polymers are often expensive or have properties other than impact resistance that make them less than optimum selections. Thus, styrene-based copolymers, and particularly polystyrene resins that are modified with rubber particles, have been investigated for use in applications requiring less brittleness. The modification of polystyrene to reduce brittleness is often referred to increasing its impact properties and thus the modified polystyrene is said to have higher impact.
These high-impact polystyrene resins, commonly referred to by the acronym HIPS, are known to be useful in the art of preparing articles with polymers wherein the application for the articles requires less brittleness than unmodified polystyrene. For example, U.S. Defensive Publication T59,011 to Smith discloses that a high impact resin can be prepared by blending from 15 to 50 parts of an impact modifier with from 85 to 50 parts of a clear crystal polystyrene. Such materials are disclosed to be useful for packaging applications.
Another method of making HIPS is to first dissolve a rubber in styrene monomer and then polymerize the monomer. Such polymers are disclosed in U.S. Pat. No. 6,569,941 to Sosa, et al. Therein, it is disclosed that styrene monomer containing a dissolved polybutadiene rubber is flowed into an elongated upflow stirred reactor containing three reaction zones, wherein the styrene monomer is polymerized to form a HIPS.
In HIPS, desirably the polystyrene is a continuous phase including a discontinuous phase of rubber particles. The size and distribution of the rubber particles in the continuous polystyrene phase can affect the properties of the HIPS. In blends of polystyrene with other materials, the distribution of the non-continuous phase in the continuous polystyrene phase is often similarly important.
Excessive heat history introduced to a HIPS composition may result in additional crosslinking of the rubber phase. Heat history may be introduced, for example, through devolatilization of the HIPS during manufacture, compounding, extrusion, or other processing of the HIPS at elevated temperatures. The resulting crosslinking of the discontinuous rubber phase may result in embrittlement of the rubber and a corresponding decrease in the impact strength of the polymer.
JP 2001-191461 discloses a rubber modified styrenic polymer obtained by polymerizing a monomer mixture based on a styrenic monomer and a methacrylic ester monomer in the presence of a rubbery elastomer. The composition may be used to prepare heat-shrinkable multilayer plastic films having good transparency and impact resistance. U.S. Pat. No. 4,782,127, which is incorporated herein by reference, discloses the use of an ester of acrylic acid, such as n-butyl acrylate, to improve melt flow properties of polystyrene.
As disclosed in PCT Application Nos. PCT/US2007/012821, PCT/US2007/012822, and PCT/US2007/012817, and U.S. Provisional Patent Application Ser. No. 60/932,790, each of which are incorporated herein by reference, microwave heating of polymers may reduce cycle times, volumetric heating with microwaves may eliminate the need for “surface” or “contact” heating, and therefore may eliminate the potentially deleterious effects of high polymer surface temperatures, such as discoloration, surface defects, and gloss gradients, among others. Volumetric heating may also eliminate the undesirable temperature gradient through a sheet thickness, such as may typically result from radiant heating of a polystyrene sheet during a thermoforming process.
Unfortunately, polystyrene and high impact polystyrene are not receptive to microwave heating, as they do not have any functional groups that will absorb microwave energy (i.e., polystyrene is transparent to microwaves). Microwave-receptive additives, such as a zeolite or water, may be used to make a polystyrene matrix susceptible to microwaves; however, this requires an additional compounding step, adding undesired heat history to the rubber phase, and these compounds may affect the clarity and mechanical properties of the HIPS. These compounding additives may be expensive, and the compounding of the same additionally increases the cost of the resulting polystyrene.
Accordingly, there exists a need for high impact polystyrenes that may be processed using microwave heating apparatus.